This application is based upon and claims benefit of priority of Japanese Patent Applications No. 2000-359838 filed on Nov. 27, 2000 and No. 2001-45293 filed on Feb. 21, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a pressure sensor detecting a pressure difference imposed on both surfaces of a semiconductor sensor chip.
2. Description of Related Art
Examples of pressure sensors of this kind are disclosed in JP-A-9-43085, JP-A-3-237332 and JP-A-11-241970. A conventional pressure sensor disclosed in those publications includes a semiconductor diaphragm-type sensor chip mounted on a case. A first pressure is imposed on a front surface of the sensor chip and a second pressure is imposed on its rear surface, and a pressure difference between the first and the second pressures is detected as an electrical signal. An electrical circuit for converting the pressure difference into the electrical signal is formed on the front surface of the sensor chip, and the rear surface of the sensor chip is pasted on the sensor case via a substrate.
The front surface of the sensor chip on which the electrical circuit is formed is covered with a protecting member made of rubber, gel or the like to protect the electrical circuit from dust or moisture. The rear surface of the sensor chip, however, is not covered. A rear passage introducing the second pressure onto the rear surface of the sensor chip is considerably narrow, compared with a front passage introducing the first pressure onto the front surface. This is because the rear surface of the sensor chip has to be pasted on the sensor case. The narrow rear passage tends to become clogged with foreign particles including dust and moisture when the pressure sensor is used in a non-ideal operating environment because the rear passage is not protected from the environment. The sensor case may be damaged by the condensed moisture in the worst case. If the rear passage is clogged with foreign particles, the pressure sensor does not function correctly.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved pressure sensor which accurately detects the pressure difference between pressures imposed on both surfaces of the sensor chip even when the pressure sensor is used in a bad atmosphere.
A semiconductor sensor chip having a front surface and a rear surface is mounted on and contained in a case. The case includes a first passage through which a first pressure is introduced and a second passage through which a second pressure is introduced. The first pressure is supplied to the front surface of the sensor chip, while the second pressure is supplied to the rear surface. A pressure difference between the first and second pressures is detected by the sensor chip and converted into an electrical signal. The pressure sensor is used, for example, as a sensor for detecting mesh-clogging of a diesel particulate filter.
A thin diaphragm is formed on the front surface of the sensor chip by making a cavity from the rear surface. The diaphragm distorts according to the pressure difference between the first and second pressures, and the distortion is converted into an electrical signal. The sensor chip is mounted on the case via a substrate connected to the rear surface of the sensor chip. The second pressure is supplied to the diaphragm of the sensor chip through a small through-hole formed in the substrate.
The front surface of the sensor chip is covered with a first protecting member to protect the sensor chip from foreign particles such as dusts and moisture contained in the gas such as exhaust gas. The rear surface of the sensor chip is also covered with a second protecting member to protect the sensor chip against the foreign particles. The small through-hole formed in the substrate is connected to a rear passage that includes a pressure passage having a relatively small cross-sectional area and a depressed portion having a large cross-sectional area. The second protecting member fills both of the small through-hole formed in the substrate and the rear passage. The second pressure is first received by a large area of the second protecting member filling the depressed portion and then transferred to the rear surface of the sensor chip through the second protecting member filling the rear passage and the small through-hole. In this manner, the rear surface of the sensor chip is protected without fail from the moisture or the dusts contained in the gas.
The first and the second protecting members are composed of a host material such as phlorosilicone gel or fluorine gel and oil added to the host material. Preferably, the oil of the same type as the host material is added to the host material. Since the second protecting member fills the small through-hole while the first protecting member covers a relatively large area, more oil is added to the second protecting member than to the first protecting member to make the second protecting member softer. For example, oil in an amount of more than 30 percents of the host material is added to the second protecting member.
Either one of the first gas introducing passage or the second gas introducing passage may be made in a funnel shape extending in a direction perpendicular to the surface of the sensor chip. Moisture condensed in the gas introducing passage can be easily discharged along a tapered wall of the funnel-shaped passage. The taper angle of the tapered wall relative to the sensor chip surface is made larger than 30 degrees, preferably, larger than 45 degrees.
According to the present invention, the rear surface of the sensor chip is protected by the second protecting member in addition to protecting the front surface by the first protecting member. Therefore, the sensor chip is surely protected against dusts and moisture contained in the gas introduced in the sensor case, and thereby the pressure sensor accurately detects the pressure supplied thereto.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.